This application claims the benefit of a Japanese Patent Application No.2001-186273 filed Jun. 20, 2001, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to peak suppression methods and data transmission apparatuses, and more particularly to a peak suppression method for suppressing a peak of a transmitting signal power in a system which transmits data by multi-carrier at a high speed so as not to deteriorate a signal-to-noise (S/N) ratio on a receiver end, and to a data transmission apparatus which transmits data by suppressing a peak of a transmitting signal.
Multi-carrier data transmission which uses multi-carrier is applicable to various transmission systems including cable transmission such as ADSL and SDSL, wireless transmission such as OFDM, and optical transmission such as WDM.
2. Description of the Related Art
In systems which transmit data via cable, wireless and optical transmission channels, there are demands to improve the data transmission rate. In addition, among cable data transmission systems, there are proposed systems which utilize an existing distribution line. There is also a distribution system which supplies power from a substation to each transformer via a 6.6 kV high-voltage distribution line, for example, and steps down the voltage to 100 V or 200 V at each transformer, so as to supply the power to homes or the like via a low-voltage distribution line. As means of effectively utilizing the last-one-mile, various data transmission systems have been proposed which utilize the low-voltage distribution line as a data transmission line.
In the data transmission system which utilizes the low-voltage distribution line, the high-voltage distribution line side utilizes an optical fiber transmission line which is set up along the high-voltage distribution line, connects this optical fiber transmission line and the low-voltage distribution line by a modem, and connects a terminal equipment and the low-voltage distribution line by a modem, so as to make the data transmission in the last-one-mile using the low-voltage distribution line as the data transmission line. In this case, operating power to the terminal equipment is supplied via the low-voltage distribution line. Hence, terminal equipment can make the data transmission using the internal modem of the terminal equipment, by simply connecting a power line of the terminal equipment to a plug socket.
In this case, an outdoor low-voltage distribution line is equivalent to an inductance of approximately 1 μH/m, for example, and an indoor low-voltage distribution line is equivalent to a capacitance of approximately 75 pF/m, for example. Accordingly, if the length of the low-voltage distribution line is 150 m, for example, such that the length of a service wire is 50 m for 30 homes, a line characteristic becomes equivalent to a connection of an inductance of 150 μH and a capacitance of 0.1125 μF. Since noise eliminating capacitors are connected to various home appliances, an impedance of the low-voltage distribution line when viewed from the modem on the optical fiber side has relatively large inductance and capacitance.
When transmitting the data using such a low-voltage distribution line, a transmitting signal having a power (PWR) versus frequency characteristic shown in FIG. 1A is transmitted from a modem of a mast or pole side. Since the low-voltage distribution line has a line characteristic including inductance and capacitance as shown in FIG. 1B, a lowpass filter characteristic is obtained. Accordingly, the low-voltage distribution line has a reception characteristic shown in FIG. 1C having a large attenuation in a high frequency region. The noise generated from the home appliances such as an inverter equipment has a relatively large power in a low frequency region. In the case of a noise level indicated by a dotted line and a received signal indicated by a solid line in FIG. 1C, the received signal becomes buried in the noise.
It is conceivable to cut the low frequency region where the noise level is high, as shown in FIG. 2A, so as to transmit the data utilizing the high frequency region where the noise level is low. However, the S/N ratio is not improved by cutting the low frequency region where the received signal level is high. For this reason, various proposals have been made to improve the S/N ratio by positively cancelling the noise. FIG. 2B shows a case where the noise in the low frequency region is cancelled. As shown in FIG. 2B, the S/N ratio can be improved as a whole since the received signal level in the low frequency region becomes higher than the noise level.
On the other hand, an orthogonal frequency division multiplexing (OFDM) system transmits the data using multi-carrier, and each carrier is selected to have an orthogonal relationship. By using the multi-carrier to make the multiplexed transmission, it is possible to allocate the carrier frequencies by avoiding a band where the noise level is large, for example. A discrete multi-tone (DMT) system also transmits the data using a plurality of carriers, and is used as an asymmetric digital subscriber line (ADSL) modulation system, for example.
A transmitting analog section of a data transmission apparatus has a structure shown in FIG. 3, for example. The transmitting analog section shown in FIG. 3 includes a digital-to-analog (D/A) converter 101, a lowpass filter (LPF) 102, a gain adjusting part 103, a line driver 104, a line transformer 105, and a coupling filter 106. A transmitting signal is converted into an analog signal by the D/A converter 101, and is eliminated of an unwanted high-frequency component in the lowpass filter 102. A line output signal level is adjusted by the gain adjusting part 103, so that the line driver 104 and the like will not saturate. The output of the gain adjusting part 103 is transmitted to a low-voltage distribution line via the line driver 104, the line transformer 105 and the coupling filter 106 which couples to the AC line.
FIG. 4 is a system block diagram showing a conceivable data transmission apparatus. This data transmission apparatus corresponds to a modem which transmits and receives data by connecting to the low-voltage distribution line. The data transmission apparatus shown in FIG. 4 includes a code converter 111, a signal generator 112, an inverse fast Fourier transform (IFFT) section 113 including a guard time (GT) adding function, a zero point inserting section 114, a roll-off filter (ROF) 115, a modulator (MOD) 116, a digital-to-analog (D/A) converter 117, a lowpass filter (LPF) 118, a transmission clock generator (TX-CLK) 119, a bandpass filter (BPF) 120, an analog-to-digital (A/D) converter 121, a demodulator (DEM) 122, a roll-off filter (ROF) 123, a reception clock distributor (RX-CLK) 124, a timing extractor (TIM) 125, a phase locked loop (PLL) circuit 126 including a voltage controlled crystal oscillator (VCXO), a noise eliminating section 127, a fast Fourier transform (FFT) section 128 including a guard time (GT) deleting function, a signal deciding section (DEC) 129, a code converter 130. The code converter 111 includes the functions of a scrambler (SCR), a serial-to-parallel (S/P) converter, a Gray code/natural code (G/N) converter, a sum computing unit and the like. On the other hand, the code converter 130 includes the functions of a parallel-to-serial (P/S) converter, a descrambler (DSCR), a difference computing unit, a natural code/Gray code (N/G) converter and the like. In FIG. 4, TX-line denotes a transmission line, RX-line denotes a reception line, SD denotes a transmitting signal, and RD denotes a received signal.
A clock signal generated from the transmission clock generator 119 is supplied to various parts within the data transmission apparatus, including the zero point inserting section 114 which receives the clock signal as a zero point insertion timing signal. The transmitting signal SD is subjected to processes including a scrambling process, a S/P conversion in correspondence with the number of carriers, G/N conversion, the sum operation to enable a difference operation at the receiving end, and the like, in the code converter 111. The signal from the code converter 111 is supplied to the signal point generator 112 which generates signal points at Nyquist intervals, and the IFFT section 113 carries out the addition of the guard time (GT) and the IFFT process. The zero point inserting section 114 inserts a zero point indicating a level 0 depending on the zero point insertion timing signal, and the roll-off filter 115 carries out a wave-shaping with respect to the output of the zero point inserting section 114. The modulator 116 subjects the output of the roll-off filter 115 to a digital modulation, and the D/A converter 117 converts the output of the modulator 116 to an analog signal. The analog signal from the D/A converter 117 is formed into a signal having a transmission band of 10 kHz to 450 kHz, for example, by the lowpass filter 118, and is transmitted to the transmission line TX-line.
The reception clock distributor 124 distributes to various parts within the data transmission apparatus a clock signal which is based on a clock signal received from the PLL circuit 126. The signal received via the reception line RX-line is supplied to the bandpass filter 120 which passes a signal having a band of 10 kHz to 450 kHz, for example. The output signal of the bandpass filter 120 is converted into a digital signal by the A/D converter 121 and is then demodulated by the demodulator 122. The roll-off filter 123 subjects the output of the demodulator 122 to a wave-shaping. The noise eliminating section 127 obtains a noise level multiplexed to the zero point position based on the clock signal received from the reception clock distributor 124, obtains a noise level of the signal point by carrying out an interpolation process, and eliminates the noise multiplexed to the signal point. The FFT section 128 deletes the guard time (GT) and carries out a conversion to a frequency region with respect to the output of the noise eliminating section 127. The signal from the FFT section 128 is judged (or decoded) by the signal deciding section 129. With respect to the output of the signal deciding section 129, the code converter 130 carries out processes such as P/S conversion, descrambling process, difference operation and N/G conversion, so as to output the received signal RD.
In the case of a data transmission via a data transmission line, such as the low-voltage distribution line and the wireless line, into which large noise mixes, the above described structure of the data transmission apparatus enables the receiving end to detect the timing of the zero point which is inserted at the transmitting end, so that the noise may be cancelled based on the noise level at the detected timing. As described above with reference to FIG. 2B, it is possible to improve the S/N ratio.
The data transmission apparatus described above carries out the data transmission by multi-carrier. Hence, if a monotone signal is transmitted by each of the multi-channels, for example, the required signal energy becomes a sum of the signal energies of the channels. For example, when making the transmission by allocating transmitting signals S to each of the carriers on the frequency axis in FIG. 5A, the signals of each of the channels are added on the time base. As a result, the signal levels of the original signal shown in FIG. 5B greatly change depending on the match or mismatch of the phase, level and the like of the signals of each of the channels. For example, signals “a” and “b” which exceed levels indicated by dotted lines in FIG. 5B become saturated signals a′ and b′ as shown in FIG. 5C due to saturation caused by the characteristic of the line driver 104 and the like. When such a saturation occurs, the distortion of the received signal becomes large, to greatly deteriorate the S/N ratio.
In this case, if the number of channels is denoted by n and a peak to average ratio (a peak value with respect to an average value) is denoted by PAR, the peak to average ratio PAR can be described by PAR=3.01+10 log n [dB]. For example, when making the transmission by multiplexing 64 channels, n=64 and PAR=+21.07 [dB]. Accordingly, it is necessary to reduce the transmission level to avoid saturation, and for example, it is necessary to reduce the gain of the gain adjusting section 103 in the transmitting section of the data transmission apparatus shown in FIG. 4.
But when the transmission level is reduced, no saturation occurs at the peak point, but the reception level is also reduced, thereby deteriorating the S/N ratio. For this reason, even if the noise elimination is made based on the zero point insertion by the structure shown in FIG. 4, the S/N ratio assumes a negative value, and it may become impossible to make a high-speed data transmission.